Amid strong demand for preventing ozone layer destruction and global warming in these days, it is imperative also in the field of air conditioning and refrigeration not only to draw back from using CFCs from the viewpoint of preventing ozone layer destruction, but also to recover alternative compounds HFCs and to improve energy efficiency from the viewpoint of preventing global warming. To meet the demand, utilization of natural refrigerant such as ammonia, hydrocarbon, air, carbon dioxide, etc. is being considered, and ammonia is being used in many of large cooling/refrigerating equipment. Adoption of natural refrigerant tends to increase also in cooling/refrigerating equipment of small scale such as a refrigerating storehouse, goods disposing room, and processing room, which are associated with said large cooling/refrigerating equipment.
However, as ammonia is toxic, a refrigerating cycle, in which an ammonia cycle and CO2 cycle are combined and CO2 is uses as a secondary refrigerant in a refrigeration load side, is adopted in many of ice-making factories, refrigerating storehouses, and food refrigerating factories.
A refrigeration system in which ammonia cycle and carbon dioxide cycle are combined is disclosed in Patent Literature 1 for example. The system is composed as shown in FIG. 11(A). In the drawing, first, in the ammonia cycle gaseous ammonia compressed by the compressor 104 is cooled by cooling water or air to be liquefied when the ammonia gas passes through the condenser 105. The liquefied ammonia is expanded at the expansion valve 106, then evaporates in the cascade condenser 107 to be gasified. When evaporating, the ammonia receives heat from the carbon dioxide in the carbon dioxide cycle to liquefy the carbon dioxide.
On the other hand, in the carbon dioxide cycle, the carbon dioxide cooled and liquefied in the cascade condenser 107 flows downward by its hydraulic head to pass through the flow adjusting valve 108 and enters the bottom feed type evaporator 109 to perform required cooling. The carbon dioxide heated and evaporated in the evaporator 109 returns again to the cascade condenser 107, thus the ammonia performs natural circulation.
In the system of said prior art, the cascade condenser 107 is located at a position higher than that of the evaporator 109, for example, located on a rooftop. By this, hydraulic head is produced between the cascade condenser 107 and the evaporator 109 having a cooler fan 109a. 
The principle of this is explained with reference to FIG. 1(B) which is a pressure-enthalpy diagram. In the drawing, the broken line shows an ammonia refrigerating cycle using a compressor, and the solid line shows a CO2 cycle by natural circulation which is possible by composing such that there is a hydraulic head between the cascade condenser 107 and the bottom feed type evaporator 109.
However, said prior art includes a fundamental disadvantage that the cascade condenser (which works as an evaporator in the ammonia cycle to cool carbon dioxide) must be located at a position higher than the position of the evaporator (refrigerating showcase, etc.) for performing required cooling in the CO2 cycle.
Particularly, there may be a case that refrigerating showcases or freezer units are required to be installed at higher floors of high or middle-rise buildings at customers' convenience, and the system of the prior art absolutely can not cope with the case like this.
To deal with this, some of the system provide a liquid pump 110 as shown in FIG. 11(B) in the carbon dioxide cycle to subserve the circulation of the carbon dioxide refrigerant to ensure more positive circulation. However, the liquid pump serves only as an auxiliary means and basically natural circulation for cooling carbon dioxide is generated by the hydraulic head also in this prior art.
That is, in the prior art, a pathway provided with the auxiliary pump is added parallel to the natural circulation route on condition that the natural circulation of CO2 is produced by the utilization of the hydraulic head. (Therefore, the pathway provided with the auxiliary pump should be parallel to the natural circulation route.)
Particularly, the prior art of FIG. 11(B) utilizes the liquid pump on condition that the hydraulic head is secured, that is, on condition that the cascade condenser (an evaporator for cooling carbon dioxide refrigerant) is located at a position higher than the position of the evaporator for performing cooling in the carbon dioxide cycle, and above-mentioned fundamental disadvantage is not solved also in this prior art.
In addition, it is difficult to apply this prior art when evaporators (refrigerating showcases, cooling apparatuses, etc.) are to be located on the ground floor and the first floor and accordingly the hydraulic head between the cascade condenser and each of the evaporator will be different to each other.
In the prior arts, there is a restriction for providing a hydraulic head between the cascade condenser 107 and the evaporator 109 that natural circulation does not occur unless the evaporator is of a bottom feed type which means that the inlet of CO2 is located at the bottom of the evaporator and the outlet of CO2 is provided at the top thereof as shown in FIG. 11(A) and FIG. 11(B).
However, in the bottom feed type condenser, liquid CO2 enters the cooling tube from the lower side evaporates in the cooling tube and flows upward while receiving heat, i.e. depriving heat of the air outside the cooling tube, and the evaporated gas flows upward in the cooling tube. So, in the cooling tube, the upper part is filled only with gaseous CO2 resulting in poor cooling effect and only lower part of the cooling tube is effectively cooled. Further, when a liquid header is provided at the inlet side, uniform distribution of CO2 in the cooling tube can not be realized. Actually, as can be seen in pressure-enthalpy diagram of FIG. 1(B), CO2 is recovered to the cascade condenser after liquid is CO2 perfectly evaporated.
Further, a refrigerating cycle using CO2 as a secondary refrigerant for refrigerating load side is adopted very often in ice works, refrigeration warehouses, and freezing works of food. In these refrigerating apparatuses, it is required to stop the operation of apparatus and to carry out defrosting and cleaning of the cooler (evaporator) at regular intervals or as needed from point of view of maintaining refrigerating capacity, sterilization, etc. When these work operation are carried out, temperature rise occurs naturally in the cooler (evaporator). So, if liquid CO2 remains in the circulation path near the cooler (evaporator), there is fear that explosive vaporization (boiling) of liquid CO2 could occur. Therefore, it is desired to withdraw the liquid CO2 remaining near the cooler (evaporator) without delay and completely.
[Patent Literature 1] Japanese Patent No. 3458310